1. Field of the Invention
This invention relates to an over-coating agent for forming fine patterns in the field of photolithographic technology and a method of forming fine-line patterns using such agent. More particularly, the invention relates to an over-coating agent for forming or defining fine-line patterns, such as hole patterns and trench patterns, that can meet today's requirements for higher packing densities and smaller sizes of semiconductor devices.
2. Description of the Related Art
In the manufacture of electronic components such as semiconductor devices and liquid-crystal devices, there is employed the photolithographic technology which, in order to perform a treatment such as etching on the substrate, first forms a film (photoresist layer) over the substrate using a so-called radiation-sensitive photoresist which is sensitive to activating radiations, then performs exposure of the film by selective illumination with an activating radiation, performs development to dissolve away the photoresist layer selectively to form an image pattern (photoresist pattern), and forms a variety of patterns including contact providing patterns such as a hole pattern and a trench pattern using the photoresist pattern as a protective layer (mask pattern).
With the recent increase in the need for higher packing densities and smaller sizes of semiconductor devices, increasing efforts are being made to form sufficiently fine-line patterns and submicron-electronic fabrication capable of forming patterns with linewidths of no more than 0.20 μm is currently required. As for the activating light rays necessary in the formation of mask patterns, short-wavelength radiations such as KrF, ArF and F2 excimer laser beams and electron beams are employed. Further, active R&D efforts are being made to find photoresist materials as mask pattern formers that have physical properties adapted to those short-wavelength radiations.
In addition to those approaches for realizing submicron-electronic fabrication which are based on photoresist materials, active R&D efforts are also being made on the basis of pattern forming method with a view to finding a technology that can provide higher resolutions than those possessed by photoresist materials.
For example, Patent Reference 1 discloses a method of forming fine patterns which comprises the steps of defining patterns (=photoresist-uncovered patterns) in a pattern-forming resist on a substrate, then coating over entirely the substrate with a mixing generating resist that is to be mixed with said pattern-forming resist, baking the assembly to form a mixing layer on both sidewalls and the top of the pattern-forming resist, and removing the non-mixing portions of said mixing generating resist such that the feature size of the photoresist-uncovered pattern is reduced by an amount comparable to the dimension of said mixing layer. Patent Reference 2 discloses a pattern forming method comprising the steps of depositing a resin, which becomes insoluble in the presence of an acid, on a substrate having formed thereon a resist pattern containing an acid generator, heat treating the assembly so that the acid is diffused from the resist pattern into said resin insoluble in the presence of an acid to form a given thickness of insolubilized portion of the resist near the interface between the resin and the resist pattern, and developing the resist to remove the resin portion through which no acid has been diffused, thereby ensuring that the feature size of the pattern is reduced by an amount comparable to the dimension of said given thickness.
However, in these methods, it is difficult to keep the in-plane temperature of the wafers uniformly by means of the heater employed in current fabrication of semiconductor devices, and therefore the heat dependency of the layers formed on the resist patterns on the wafers is as great as ten-odd nanometers per degree Celsius, and this leads to the problem of occurrence of significant variations in pattern dimensions. Furthermore, there are other problems such as occurrences of defects due to the layers formed on the resist patterns and cracks in the layers themselves. In addition, these methods tend to be highly resist-dependent because the degrees of reducing pattern dimensions largely differ due to the contents of acids in the resists.
Another approach known to be capable of reducing pattern dimensions is by fluidizing resist patterns through heat treatment and the like. For example, Patent Reference 3 discloses a method comprising the steps of forming a resist pattern on a substrate and applying heat treatment to deform the cross-sectional shape of the resist pattern, thereby defining a fine pattern. In addition, Patent Reference 4 discloses a method comprising the steps of forming a resist pattern and heating it to fluidize the resist pattern, thereby changing the dimensions of its resist pattern to form or define a fine-line pattern.
In these methods, the wafer's in-plane heat dependency of the resist pattern is only a few nanometers per degree Celsius and is not very problematic. On the other hand, it is difficult to control the resist deformation and fluidizing on account of heat treatment, and resist pattern profile deteriorate, such as non-rectangularization, pattern-collapse, so it is not easy to form a uniform fine-lined resist pattern in a wafer's plane.
An evolved version of those methods is disclosed in Patent Reference 5 and it comprises the steps of forming a resist pattern on a substrate, forming a stopper resin on the substrate to prevent excessive thermal fluidizing of the resist pattern, then applying heat treatment to fluidize the resist so as to change the dimensions of its pattern, and thereafter removing the stopper resin to form or define a fine-line pattern. As the stopper resin, specifically, polyvinyl alcohol is employed. However, polyvinyl alcohol alone is not highly soluble in water and cannot be readily removed completely by washing with water, introducing difficulty in forming a pattern of good profile. The pattern formed is not completely satisfactory in terms of stability over time. In addition, polyvinyl alcohol cannot be applied efficiently by coating, and when it is employed to a substrate formed thereon resist patterns having a different spacing, i.e., a wide spacing and a narrow spacing, between adjacent resist patterns, in-plane-uniform amounts of shrinkage cannot be obtained. Because of these and other problems, the method disclosed in Patent Reference 5 has yet not to be adopted commercially.
For solving these prior-art problems, the present applicant has proposed a technique directed to an over-coating agent for forming fine patterns and to a method of forming fine patterns in Patent References 6-11. The technique shown in these Patent References 6-11 has made it possible to form fine-line patterns that satisfy pattern dimension controllability, good profile and other necessary properties for semiconductor devices. These techniques are quite efficient in defining fine-line patterns on the substrate especially having photoresist patterns thereon in the same pitches, but there have not been made investigations for the substrate having photoresist patterns having thereon in the different spacing or “pitch” between adjacent photoresist patterns, such as “isolate” patterns having wide spacing between adjacent photoresist patterns, and “dense” patterns having narrow spacing between adjacent photoresist patterns.
In forming fine-line patterns or reducing the spacing between adjacent photoresist patterns on the substrate where the photoresist patterns have different pitches of both isolate patterns and dense patterns, the thermal-shrinkage behavior of the over-coating agent tends to be pitch-dependent of the photoresist patterns, and it was difficult to control to obtain the equal amount of thermal shrinkage of the over-coating agent without regard to differences of pitches of photoresist patterns. Therefore, reducing amount of the patterns differ between the photoresist patterns formed by isolate pitches and the ones formed by dense pitches. Because of this, it was conducted to control the temperature of the heat treatment and time of the heat treatment of the over-coating agent (coating film) in accordance with the degree of isolation or density of the pitches so as to obtain pitch-independently the equal amount of thermal shrinkage of the coating. In this technique, however, the amount of the shrinkage is apt to vary due to a lot-to-lot variability of the over-coating agent.
It is desirable that, even when various patterns differing in the pattern dimension and the line-to-line distance exist on one substrate, such as densely-pitched ones or isolated-pitched ones, all the patterns could enjoy the same degree of thermal shrinkage of the over-coating agent.
Patent Reference 1: JP5-166717A
Patent Reference 2: JP5-241348A
Patent Reference 3: JP1-307228A
Patent Reference 4: JP4-364021A
Patent Reference 5: JP7-45510A
Patent Reference 6: JP2003-084459A
Patent Reference 7: JP2003-084460A
Patent Reference 8: JP2003-107752A
Patent Reference 9: JP2003-142381A
Patent Reference 10: JP2003-195527A
Patent Reference 11: JP2003-202679A